The answer to this depends to some degree on what we mean by bias.
On the one hand, Americans' response to the war was somewhat colored by bias against Germans. There had been a great deal of immigration in the late 1800s and early 1900s and people were somewhat leery of all immigrants, including Germans. This bias against Germans in America translated to some degree to a bias against Germany in the war.
But the main reason for America's attitude in this war was economic. If this counts as bias (clearly, this is not ethnic or racial bias), then bias played a very important role. America traded much more with England (particularly as the war went on) than it did with Germany. This led many Americans to be biased in favor of England because that was in line with their economic interests.
Therefore, we can say that bias played a moderate to great role in Americans' response to the war, depending on how you define bias.
The U.S. government was quite biased against Germany during WWI. Early on in the war, the British navy cut the German transatlantic cable; this act made sure that only pro-Allied news concerning the war left Europe. German use of unrestricted submarine warfare was considered barbaric, but no one in America looked at the humanitarian damage wrought by the mining of the North Sea, which caused starvation conditions in Germany before the end of the war. Also, the German army was considered barbaric for its actions against Belgium, though there was little evidence of war crimes there. British artists recruited for the war effort made Germany appear sinister; many even described the German army as "Huns," thus evoking the image of marauding armies bent on destroying civilization.
The Allies wanted to create an anti-German bias in the U.S. because they needed American supplies and ultimately American manpower to win the war. The Allies realized that gaining American support for their side would not be a sure thing, since America was not in danger and Irish-Americans and German-Americans might side with the Central Powers. While joining the war on the Allied side had economic benefits for American businessmen, the use of bias helped to get American hearts and minds on the side of the Allied powers.